Comments 0

Document transcript

Abstract—The proposed Physiological Signal ProcessingLaboratory incorporates important new concepts to further itsutility as a vehicle for biomedical engineering educational use.The Laboratory incorporates the physical construction, testingand analysis of eight signal processing circuit modules,introduced as lessons. Each module can be characterizedthrough measurement with a BIOPAC MP35 data acquisitionsystem and a student-built square wave generator. The modulesare combined sequentially to create a sophisticated andfunctional electrocardiogram (ECG) amplification andprocessing system. By the final lesson, the completed ECG SignalProcessor will provide meaningful outputs from signals sourcedfrom the student’s own body. Through the application of asingle, easy-to-use data acquisition system and associatedsoftware to a breadboard circuitry laboratory, students canbuild, test and analyze signal processing modules, verify theirperformance against mathematical simulation using graphicalcomparisons, combine modules, collect physiological signalssourced from their own bodies, and evaluate the results. Bydeveloping the complete ECG Signal Processor, module bymodule (as eight lessons), students develop an understanding ofsystem design and development methodologies. In addition,when collecting data directly from their own bodies, students’curiosity is stimulated to create an environment more amenableto inquiry-based learning.

The proposed Physiological Signal Processing Laboratoryfor biomedical engineering (BME) education is an evolutionof the BME Laboratory introduced in 2003 by BIOPACSystems, Inc. This evolution incorporates new understandingresulting from extensive teaching laboratory use of theprevious Laboratory. The evolved elements in this Laboratoryinclude improvements in: circuit stimulation methodology;filter analysis techniques; module development strategy(practical order for an educational setting); laboratory support(written materials provide clear introductions and moduletheory of operation).Practical laboratory experience, guided by a modulardevelopment approach, is important and meaningful

Inquiry-based learning is a student-centered, activelearning approach focused on questioning, critical thinkingand problem solving. Inquiry-based education is characterizedby a learning environment structured to create opportunitiesfor students to be engaged in active learning based upon theirown questions. Involvement in learning implies the studentsare developing capabilities and perceptions that permit them tolook for solutions to problems during the course of acquiringknowledge.Physiological Signal Processing Laboratoryfor Biomedical Engineering Education

Steve Carmel and Alan J. Macy

BIOPAC Systems, Inc., 42 Aero Camino, Goleta, CA, USA (www.biopac.com)This Laboratory addresses the challenge of teachingcertain fundamentals of physiological signal processing relatedto biomedical engineering and promotes the concept thatstudent inquiry implies involvement that leads tounderstanding. The involvement must result in developedcapabilities and perceptions, for the students’ inquiries to bemeaningful and lead to further understanding, for inquiry-based learning to thrive.The Laboratory incorporates several concepts critical fordeveloping student capabilities and perceptions to supportinquiry-based learning in the area of physiological signalprocessing. The Laboratory incorporates the physicalconstruction and testing of a variety of simple signalprocessing circuit modules, each introduced as a lesson. Thecharacteristics of each module can be easily determinedthrough measurement with a BIOPAC MP35 data acquisitionunit (Fig. 1) and associated Laboratory software (BSL PRO).Over the eight Laboratory lessons, students progressivelybuild, module-by-module, a complete physiological signalprocessing system. By the end of the lesson series, studentscan employ the electrical signal detected from their own hearts(via skin surface potentials) as the signal source for the ECGSignal Processor, which provides useful outputs and featuressuch as clinical ECG, hum rejection, and QRS wave detector.In the process of building this system, students learn:• practical issues associated with signal processingmodule (circuit) construction and testing• the importance of stable signal generation andmeasurement for circuit analysis• tools and methods useful for circuit analysis,including transfer functions and circuit simulation• the relationship of any single processing module tothe complete system

Fig. 1: BIOPAC MP35 and SS39L BreadboardThe BIOPAC MP35 data acquisition unit, BSL PROsoftware and SS39L Breadboard are sufficient to permitstudents to complete the Laboratory. The BIOPAC MP35 unitis used to perform one or two channel storage scopemeasurements and supply power to the breadboard. TheMP35 unit is certified to IEC60601-1 medical electrical safetystandards and provides a double fault protected, galvanicallyisolated, current-limited ±5 Volt power supply to the SS39Lbreadboard. Students use the BSL PRO software for circuitdata collection, analysis, and simulation, thus reducing theamount of time the teacher needs to spend on softwaretraining.

Fig. 2: Lab 1 Square Wave Generator

II. METHODOLOGY

The Laboratory is modular and creates a foundation thatempowers students to create different types of physiologicalsignal processing systems beyond the assigned ECG SignalProcessor. The signal processing circuit modules introducedin this Laboratory can be combined in a variety of ways tobuild a number of different real-world physiological signalprocessing systems, such as amplifiers and processors forsignals originating from the muscles (EMG), eyes (EOG),stomach (EGG), and brain (EEG).Fig. 3: Lab 2 Classic Instrumentation Amplifier

The signal processing circuit modules are fundamentalprocessors, largely orthogonal in practical operation.Depending on how modules are combined and modified,systems using similar modules can perform considerablydifferent physiological processing operations. For example,the detector topology for R-wave detection in the ECGbecomes (with slight modification) an Alpha wave indicatorwhen recording the EEG.Fig. 4: Lab 3 High Pass Filter

Signal processing circuit modules are introducedsequentially to students as lessons. Students build the moduleson a breadboard, evaluate the circuit module characteristicsand compare results to mathematical simulation using theBIOPAC MP35 data acquisition unit and BSL PROLaboratory software. Comparisons between collected andsimulated data can be performed in real time and in agraphical manner.Fig. 5: Lab 4 Positive Gain Block & Low Pass Filter

The Laboratory introduces eight fundamental signalprocessing circuit modules (lessons) to the student (Figs. 2-9)and culminates in an ECG Signal Processor (Fig. 10 )—ECGAmplifier with Hum Rejection and QRS Detector.The students begin the Laboratory series by building,testing and analyzing a Square Wave Generator (Fig. 2). Thestudents use this generator to help them analyze eachsubsequent module. The generator has a high and low leveloutput suitable for testing the amplifiers, filters, and functionblocks in the complete ECG Signal Processor.Fig. 6: Lab 5 Notch Filter

Several of the Laboratory sessions revolve around filterdesign, construction, and testing (Figs. 4-7 and 9). Filtercutoff frequencies can be measured by employing a number ofdifferent methods. By measuring the filter’s effect on “sag” or“tilt” for an input square wave, the filter’s high pass responsecan be determined.Fig. 7: Lab 6 Single Frequency Band Pass Filter

Fig. 8: Lab 7 Absolute Value Converter Fig. 9: Lab 8 Low Pass Filter

Fig. 10 ECG Signal ProcessorBy measuring the filter’s effect on rise time of an inputsquare wave, the filter’s low pass response can be estimated.In addition, by stimulating the filter with a square wave, andusing the BSL PRO software to perform a derivative on thefilter’s square wave response and then perform an FFT on thederivative result, students can produce magnitude and phaseplots of the filter’s transfer function.As an additional teaching aid, the BSL PRO software candirectly emulate (simulate) each signal processing circuitmodule, via simple software controls (Figs. 11-12). Thephysical biquad filters in the Laboratory (low pass, high pass,band pass, and notch) can be simulated in the BSL PROsoftware as 2ndorder IIR filters, operable in real time or post-processing. Equivalent simulations are available for nonlinearsignal processing circuits, such as the Absolute ValueConverter. Expression calculations are suitable for simulatingdifferential or single ended amplifiers.Before any lesson, the instructor can set up the Laboratorysoftware using a BSL PRO Template. Template files are usedto preconfigure the BIOPAC MP35 data acquisition system fora particular lesson.

Fig. 11: Software Circuit Simulation Controls

Fig. 12: Compare Actual to Simulated ResultsFor example, Templates can be used to configure two ofthe BIOPAC MP35 unit input channels as synchronizedstorage scope inputs, with a sampling rate of 10kHz and arecording time of 30 seconds.Templates are used to set up acquisition modes in theMP35 system, such as: sampling rate, acquisition length, andnumber of channels. Templates include a text “Journal” thatcan be used to present laboratory instructions and proceduresto the student and to record results. For advanced teachingapplications, Templates can easily be set to include additionalreal and simulated processing modules.

III. RESULTS

The results of a simulation that identifies the nature of thesignal at the output of each processing circuit in the ECGSignal Processor are illustrated in Fig. 12. In this graph, a realECG signal is amplified and then processed solely (insimulation) by the BSL PRO software. The data shown at theoutput of each simulated stage is presented in the same wayand is visually identical to the actual measurement atrespective points in the ECG Signal Processor physicallyconstructed on the breadboard.The Laboratory introduces fundamental physiologicalsignal processing circuit modules sequentially, as lessons,allowing students to grasp the performance of each beforeproceeding to the following module. This method establishesa strong foundation for students to design and constructprocessing systems on their own.

IV. DISCUSSION

When students collect data directly from their own bodies,the process stimulates their curiosity and gives them morecontrol over their learning by allowing them to test and retestto more fully understand the steps involved in scientificinquiry.Lab 2Lab 3Lab 4Lab 5Lab 6Lab 7Lab 8The “building-block” nature of the signal processingcircuit modules encourages students to think of novelconnection topologies between the various modules, in serviceto the principles of inquiry-based learning.